In 1835 Mexico decrees an end to the immigration of Anglos into its Texas Province, refusing the settlers demands for autonomy. The "Texian" settlers capture San Antonio and declare independence but are defeated by the Mexican Army under Santa Anna at the Alamo. A Texian force under Sam Houston then attacks Santa Anna at San Jacinto and drives him back across the Rio Grande.While the Texians proclaim their republic, Mexico continues to see Texas as a province under rebel control.

In 1845, invoking America's "manifest destiny", President Polk offers statehood to the Texians, sends a force under Zachery Taylor to the Rio Grande and declares war when Mexico crosses the river in response. The US steadily pushes the Mexicans southward in several battles. A western front is begun in 1846 when Gen. Kearny subdues the lightly defended New Mexico Territory, then marches on to California to support a "bear flag" rebellion begun by freelance officer John Fremont. The US Navy under Sloat sails into California ports claiming them for the US. Resistance to the occupation by landed Mexican "Californios" is put down by Sloats' and Kearnys' forces. A third front is opened as Winfield Scott takes the coastal city of Vera Cruz, then pushes the Mexican Army in a series of battles back into Mexico City, where Santa Anna surrenders. Santa Anna is exiled. Vast areas of territory are added to the Union, including, for better or worse, the future states of California and Texas